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Material World
Process

Contact process

Oxidising sulfur dioxide over a vanadium catalyst to make sulfuric acid — the largest-tonnage chemical process in the world.

Sulfuric acid is made in three steps: burn sulfur to sulfur dioxide, oxidise that to sulfur trioxide, and absorb the trioxide into acid. The middle step is the hard one, and it is where the process gets its name — the oxidation happens on contact with a solid vanadium pentoxide catalyst rather than in the gas phase.

The last step is counter-intuitive and is the detail that defines plant design. Sulfur trioxide is not added to water: the reaction is so violent that it produces an unmanageable mist of acid droplets that scrub out poorly and escape the stack. It is absorbed into existing concentrated sulfuric acid to make oleum, and the oleum is then diluted.

Uses

All industrial sulfuric acid, which is to say most fertiliser: phosphate rock is treated with it to make the phosphoric acid and superphosphate that agriculture runs on.

Beyond that the acid goes into leaching copper, zinc and uranium from low-grade ore, pickling steel before coating, petroleum refining, and detergents and dyes. Sulfuric acid production tracks industrial activity closely enough that economists have used it as an index, which makes this process a reasonable proxy for industrialisation itself.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • Sulfur — element · burned to sulfur dioxide, increasingly recovered from oil and gas rather than mined — and including the sulfur dioxide captured from ore roasting, which turns a smelter's principal pollutant into a feedstock

produces

  • Sulfuric acid — compound · absorbed into existing acid as oleum, then diluted — never added to water directly

is used in

  • Chemical manufacture — industry · the route to sulfuric acid, and one of the oldest continuously operated industrial catalytic processes

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Contact process back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Contact process → takes as input (burned to sulfur dioxide, increasingly recovered from oil and gas rather than mined — and including the sulfur dioxide captured from ore roasting, which turns a smelter's principal pollutant into a feedstock) → Sulfur → is extracted from (historically the principal source of sulfur for sulfuric acid) → Pyrite
  • Contact process → takes as input (burned to sulfur dioxide, increasingly recovered from oil and gas rather than mined — and including the sulfur dioxide captured from ore roasting, which turns a smelter's principal pollutant into a feedstock) → Sulfur → is produced by (recovered from the hydrogen sulfide stripped out of natural gas and crude oil — which is now almost the whole of world supply) → Claus process

Downstream — what it becomes

  • Contact process → produces (absorbed into existing acid as oleum, then diluted — never added to water directly) → Sulfuric acid → is used as (treating phosphate rock to make phosphoric acid and superphosphate) → Fertiliser
  • Contact process → is used in (the route to sulfuric acid, and one of the oldest continuously operated industrial catalytic processes) → Chemical manufacture
  • Contact process → produces (absorbed into existing acid as oleum, then diluted — never added to water directly) → Sulfuric acid → is used in (produced in greater tonnage than any other manufactured chemical, and a country's consumption of it was once used as an index of its industrial development) → Chemical manufacture